The present invention relates to disk apparatus and more particularly, to a disk apparatus for controlling a fine motion actuator of high response and small stroke and a coarse motion actuator of low response and large stroke by cooperating them with each other.
A technique of two-stage actuator has hitherto been known in which a coarse motion actuator constructed of a voice coil motor and a fine motion actuator constructed of, for example, piezoelectric devices are used in combination to position a magnetic head with high accuracy. For example, JP-A-4-368676 discloses a positioning control technique for realizing the high bandwidth of a control system by cooperating a coarse motion actuator control system with a fine motion actuator control system.
The disclosure of JP-A-4-368676 is, however, limited to only the structure of the two-stage actuator control system and fails to give a description of a design technique of a control system suitable for high bandwidth of the control system.
The magnetic disk has been increased in density year by year and speed-up of disk rotation leads to the need to increase the data transfer speed. At present, the disk rotation is of about 7200 rpm but in future, the advent of an apparatus having a disk rotation speed exceeding 12000 rpm will be expected. As the disk rotation speed increases, the frequency region in which there exist error components of disturbance components, especially position disturbance DD attributable to disk flutter and windage disturbance is broadened. Presently, a position disturbance for the 7200 rpm disk rotation exists at frequencies of about 500 Hz to 2 kHz and therefore, in the case of a control system having a band of 500 Hz, the position disturbance is amplified. If the band is increased from 500 Hz to, for example, 2 kHz, then the position error disturbance can be compressed to a great extent. With the disk rotation speed increased, however, the frequency region of the position disturbance also increases. For example, in an apparatus having a disk rotation speed of 15000 rpm, the band increases from 2 kHz up to 4 kHz as well known in the art.
A two-stage actuator system designed in accordance with the two-stage actuator control system disclosed in JP-A-4-368676 has a sensitivity function (disturbance compressive characteristic) exhibiting a characteristic in which disturbances are compressed within a band of 2 kHz or less but conversely, the individual disturbances are amplified at frequencies of 2 kHz or more.
Consequently, even when the band is increased or raised in the conventional two-stage actuator control system, the frequency band within which the position disturbances are present increases as the disk rotation increases and the reduction of positioning error expected by the improved band cannot be realized. Conceivably, by improving the mechanical system, various disturbance components can be reduced but disadvantageously, such an expedient leads to an increase in costs of the apparatus.
Accordingly, the problem of highly accurate positioning technique in the two-stage actuator control system is important and must be solved.
A first object of the present invention is to provide disk apparatus and positioning control system which can realize a sensitivity function (disturbance suppressive characteristic) for preventing disturbances from being amplified at all frequencies concerned in a two-stage actuator system having a coarse motion actuator and a fine motion actuator.
A second object of the invention is to provide a system in which the fine motion actuator control system can be controlled by an analog circuit without causing lagging in operation in the two-stage actuator system. With this system, the sensitivity function (disturbance suppressive characteristic) of fine motion actuator control system capable of preventing the disturbances from being amplified at all of the frequencies can be realized. Further, even in the whole two-stage actuator control system, a sensitivity function capable of preventing the disturbances from being amplified at all of the frequencies can be realized.
According to the present invention, an ideal two-stage actuator control system capable of reducing disturbance components at all of the frequencies can be constructed so as to attain highly accurate positioning of the head. Through this, the track pitch distance can be narrowed and therefore, a large amount of data can be recorded on one disk surface, thereby increasing the capacity of the apparatus.
To accomplish the above objects, in a head positioning apparatus comprising a coarse motion actuator having a stroke for permitting a head for recording/reproduction of information to operate over the whole of a disk, a controller for coarse motion adapted to drive the coarse motion actuator, a fine motion actuator movable by the coarse motion actuator and having its stroke limited to a small value and a controller for fine motion adapted to drive the fine motion actuator, a position difference signal calculated by comparing a head position signal obtained through detection of the position of the head with a target position is fed back to both the coarse motion controller and the fine motion controller so as to partly increase the gain of the coarse motion controller within a frequency region exceeding a positioning control band of the fine motion actuator. Also, a gain characteristic of a sensitivity function indicative of a disturbance suppressive characteristic in a signal path ranging from the target position to the position difference signal is rendered to be 1.4 or less at all of the frequencies.
Further, where the coarse motion actuator is represented by PV, the coarse motion controller is represented by CFBV, the fine motion actuator is represented by PP and the fine motion controller is represented by CFBP, the gain of a sensitivity function 1/(1+PVxc2x7CFBV) of coarse motion loop indicative of a disturbance compressive characteristic of the coarse motion actuator and coarse motion controller is rendered to be 1 or less within a frequency region in which the gain of a sensitivity function 1/(1+PPxc2x7CFBP) of fine motion loop indicative of a disturbance compressive characteristic of the fine motion actuator and fine motion controller is 1 or more.
The apparatus further comprises means for detecting a displacement of the fine motion actuator and means for generating a continuous head position signal from a high-frequency component of the continuous displacement of the fine motion actuator and a low-frequency component of the discrete position difference signal, and the continuous head position signal is fed back to the fine motion controller. Also, a gain characteristic of a sensitivity function indicative of a disturbance suppressive characteristic in a signal path ranging from the target position to the position difference signal is rendered to be 1.4 or less at all of the frequencies.
Furthermore, a peak frequency of the sensitivity function indicative of the suppressive characteristic of the fine motion actuator and fine motion controller is rendered to be substantially coincident with a frequency of a main resonance peak of the coarse motion actuator.
Then, the apparatus further comprises means for making an operation quantity to the fine motion actuator zero, means for adding an output of the coarse motion controller and an output of coarse motion disturbance input means and means for measuring a sensitivity function indicative of a disturbance suppressive characteristic in a signal path ranging from a coarse motion disturbance input signal to a signal after the addition, and a gain characteristic of the measured sensitivity function of the coarse motion control system is rendered to be 1 or less within a low-frequency region and also within a high-frequency region.
FIG. 1 is a diagram showing hardware construction of a disk apparatus according to a first embodiment of the invention.
FIG. 2 is a block diagram showing an example of construction of a following control system in the first embodiment.
FIG. 3 is a block diagram showing another example of construction of the following control system in the first embodiment.
FIG. 4 is a graph showing a frequency characteristic of a sensitivity function based on a control technique in the first embodiment.
FIG. 5 is a flow chart showing design steps based on the control technique in the first embodiment.
FIG. 6 is a graph showing a frequency characteristic of the coarse motion controller based on the control technique in the first embodiment.
FIG. 7 is a graph showing a frequency characteristic of a closed loop system of coarse motion loop based on the control technique in the first embodiment.
FIG. 8 is a block diagram showing the construction of a system for measuring the sensitivity function in the disk apparatus of the first embodiment.
FIG. 9 is a block diagram showing hardware construction of a disk apparatus according to a second embodiment of the invention.
FIG. 10 is a graph showing a frequency characteristic of a sensitivity function based on a control technique in the second embodiment of the invention.
FIG. 11 is a graph showing a frequency characteristic of a sensitivity function of a positioning control system of two-stage actuator according to a conventional technique.
FIG. 12 is a block diagram showing the construction of a positioning control system of a coarse motion actuator in the conventional technique.
FIGS. 13A and 13B are graphs showing a frequency characteristic of the sensitivity function of the coarse motion actuator based on the conventional control technique.